Research Needs for Magnetic Fusion Energy Sciences - US Burning ...

tHEME 2: cREatiNG PREdictaBLE, HiGH-PERFoRMaNcE,
StEady-StatE PLaSMaS
introduction
ScopE aNd FocuS
This Theme corresponds directly to Theme a of the Priorities, Gaps and opportunities Report 1 , a
key resource document for ReneW. its overall goal, as noted in that report, is, “The state of knowledge
must be sufficient for the construction, with high confidence, of a device that permits the creation of sustained
plasmas that meet simultaneously, all the conditions required for practical production of fusion
energy.”
This is an exceptionally broad and challenging goal, encompassing a large fraction of the current effort
in the Us magnetic Fusion energy sciences program. as described in the preceding chapter on
Theme 1, iteR will be an enormously important and necessary step toward obtaining this knowledge
base, enabling for the first time the production and study of burning plasmas, in which more
energy is produced by fusion than is used to heat them. however, it must be recognized that iteR
is not designed to be a prototype for “practical production of fusion energy.” such a demonstration
fusion reactor, often referred to as demo, to lead to economic and attractive fusion energy, would
need advances in several key areas. two in particular are the foci of Theme 2, as represented by its
title:
High fusion power density: The needed system size for a given fusion power output is determined
by the fusion power density P fus in the burning core. This is the main measure of “high performance.”
P fus is directly proportional to the square of the plasma pressure, p 2 ∝ n 2 t i 2 , where n
is the density and t i the ion temperature. iteR is designed to achieve near optimal temperatures
for fusion cross-sections; hence, the main avenue for improvement is increasing the plasma density.
The fusion power is also proportional to b 2 b 4 , where b=2m 0 p/b 2 is the plasma pressure normalized
to magnetic pressure, and b is the magnetic field. improvements can thus be realized by
increasing b, which is limited by plasma stability and dependent on magnetic configuration, and/
or by increasing magnetic field b.
Steady-state: Rather than operating in a pulsed mode, driven inductively by a transformer, a fusion
device should be sustained for indefinite durations, with high reliability. For tokamaks, this
means current must be provided and sustained by a combination of internally generated “bootstrap”
current (driven by gradients in the density and temperature profiles) and externally driven
currents (using microwaves or neutral beams). other magnetic configurations, such as stellarators,
sustain plasmas without large current, through 3-d (non-axisymmetric) magnetic fields.
in order not to use too large a fraction of the electricity produced for plasma sustainment, the
amount of required external heating and current drive must be modest.
1 “Priorities, Gaps and Opportunities: Towards A Long-Range Strategic Plan For Magnetic Fusion
Energy” Report to FESAC Oct 2007, chaired by M. Greenwald.
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